Rim and Tire Size Comparison Calculator
Compare your current tire setup against a new one. Get diameter change, speedometer error, and clearance impact instantly.
Current Tire Size
New Tire Size
How to read a tire size
A tire code like 225/45R17 contains three key values:
- 225 = tire width in millimeters.
- 45 = aspect ratio, meaning sidewall height is 45% of tire width.
- 17 = wheel (rim) diameter in inches.
When you change wheels or tires, you often want to keep the overall tire diameter close to stock. That helps protect speedometer accuracy, drivetrain behavior, and clearance inside the wheel well.
What this rim and tire size calculator gives you
This tool compares your current and proposed sizes and outputs:
- Sidewall height
- Overall tire diameter
- Circumference
- Revolutions per mile
- Speedometer difference at a chosen speed
- Ride height (ground clearance) change
As a quick rule of thumb, many drivers try to stay within ±3% overall diameter change, while ±1% is usually considered excellent.
Formula basics
1) Sidewall height
Sidewall height (mm) = width × (aspect ratio ÷ 100)
2) Overall diameter
Overall diameter (mm) = (rim diameter × 25.4) + 2 × sidewall height
3) Circumference
Circumference (mm) = π × overall diameter
4) Speedometer effect
If your new tire has a larger circumference, your actual speed will be higher than indicated. If smaller, your actual speed will be lower.
Rim and tire fitment tips
1) Keep diameter close to OEM
Large diameter differences can affect ABS, traction control, and transmission shift behavior on some vehicles.
2) Verify wheel width compatibility
A tire width should match the wheel width range recommended by the tire manufacturer. This calculator compares dimensions, but final fitment should always be confirmed with tire specs.
3) Check physical clearances
- Fender and liner clearance during full suspension travel
- Inner clearance to strut, spring perch, and control arms
- Steering lock-to-lock rub points
4) Consider load and speed ratings
Never go below the vehicle's required load rating. For safety and legality, maintain suitable speed rating and inflation pressure.
Plus sizing example
Suppose you move from a 17" wheel to an 18" wheel. To keep total diameter similar, the tire sidewall usually needs to get shorter. A common shift might look like:
- From: 225/45R17
- To: 235/40R18
That often keeps diameter close while improving steering response and appearance. Ride comfort may become slightly firmer due to reduced sidewall height.
FAQ
How much diameter difference is safe?
Many enthusiasts use ±3% as a practical limit. Closer to 0% is better for preserving factory behavior.
Will a wider tire always improve grip?
Not always. Grip depends on tire compound, temperature, alignment, and wheel width in addition to tread width.
Does this calculator account for wheel offset?
No. Offset, bolt pattern, center bore, and brake clearance are separate fitment checks and should be verified before purchasing wheels.
Final note
Use this calculator as a planning tool before buying new wheels and tires. For final decisions, always confirm with your vehicle manual, tire manufacturer data sheets, and a professional installer.